Method for determining a monitored set of cells associated with an active set of cells

ABSTRACT

The present invention is related to cellular radio communication network and is a method for determining a Monitored set (MS) of cells associated with an Active set (AS) of cells by a particular ranking of the neighbouring cells associated with each of the cells in the Active set. A neighbouring cell list is created which contains for each active cell its neighbouring cells. These cells can be in a random order and can include other active cells. A scanning operation is done starting with the neighbouring cell (C 12  or C 31 ) on top of the list which belongs to the strongest active cell (C 11 ), whereby this cell will be included in the Monitoring set if certain conditions (3,4; respectively, FIG.  4 ). Next, the list associated with the next strongest active cell (C 12 ) is considered. This scanning is repeated until the maximum size of the Monitoring set has been reached.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method in a cellular radiocommunication network. More in particular, the invention relates to amethod for determining a Monitored set of cells associated with anActive set of cells by a particular ranking of neighbouring cellsassociated with each of the cells in the Active set.

DESCRIPTION OF RELATED ART

In a typical cellular radio communication system, a geographical areaserved by a cellular radio communication network is divided into cellareas in which radio base stations, also sometimes referred to as basetransceiver stations, provide radio coverage to mobile stationsoperating in said cell areas. The mobile stations, which e.g. may beportable, pocket, hand-held or car mounted, enables mobile station users(mobile subscribers) to communicate voice, data and/or multimediainformation via the cellular radio communication network. Each radiobase station may be equipped to service one or more cells.

A mobile station may be assigned a radio communication channel dedicatedfor communication between the mobile station and the cellular networke.g. when receiving or making a phone call. In traditional cellularradio communication systems, such as e.g. Global System for Mobilecommunication (GSM), Personal Digital Cellular (PDC) and DigitalAdvanced Mobile Phone System (TDMA), he dedicated radio communicationchannel is served by a single cell referred to as the serving cell.During the phone call it may become necessary to serve the call usinganother cell e.g. due to the mobile station moving into an other cell. Anew dedicated radio communication channel is then established in theother cell for serving the phone call, while the old dedicated radiocommunication channel is released. This process of changing dedicatedradio communication channels during an ongoing call is referred to ashandoff or handover. Due to there being a short interruption ofcommunication as the mobile station switches from one dedicated radiocommunication channel to another, this type of handover is sometimesreferred to as hard handover or “break before make”.

In more recent cellular radio communication systems using so called CodeDivision Multiple Access (CDMA) technology, e.g. cdmaOne or IS-95 andthe UTRAN FDD mode (also referred to as Wideband CDMA), a dedicatedradio communication channel need not only be supported by a singleserving cell at each given moment of time, but may on the contrary besupported by several cells using so called macrodiversity which providesincreased radio transmission quality. The set of cells currently servingthe dedicated communication channel is referred to as the Active set.Due to the possibility of using more than one cell to serve a dedicatedradio communication channel, these cellular radio communication systemsalso enables so called soft handover, sometimes referred to as “makebefore break”, wherein the mobile station continues to communicate withthe cellular network via the old cell at least until the dedicated radiochannel is established also in the new cell.

In order to support handover, both in traditional systems and systemsusing CDMA, mobile stations are required to perform measurements ondownlink transmissions, i.e. from the cellular network to the mobilestations, in both the serving cell/Active set as well as neighbouringcells to the serving cell/Active set. The measurement results arereported back to the cellular network and are used for making decisionson which cell is best suited to serve the respective mobile station.

The mobile stations have limited capacity for performing the downlinkmeasurements, thus if a mobile station is ordered to perform downlinkmeasurements on transmissions in too many cells, the quality, andthereby the reliability of the measurements reported to the network, isreduced and/or it takes longer time to produce measurement results whiche.g. may cause a loss of communication in a situation where the radioenvironment changes quickly such as when the mobile station turns arounda street corner at high speed. On the other hand, if not all neighborcells to the serving cell/Active set are measured, there is a risk thatthe most suitable cell, i.e. the cell where communication between themobile station and cellular network can be maintained using minimumradio transmission power, is never considered as a candidate forhandover. To be able to use minimum transmission power for maintainingcommunication is important since this way battery operated mobilestations can operate for longer periods of time without having to berecharged. To be able to use minimum transmission power is also veryimportant in so called interference limited systems, such as systemsbased on CDMA-technology, wherein using minimum transmission powercauses reduced interference levels which translates into improved systemcapacity.

Since when employing macrodiversity/soft handover, the Active setserving a mobile station may include several cells, e.g. up to 6 cellsin UTRAN FDD mode, which each has a plurality of neighbor cells, theabove illustrated problem of the number of neighbor cells associatedwith cells in an Active set exceeding the number of neighbor cells forwhich mobile stations have adequate capacity to perform downlinkmeasurement appears frequently in cellular communication systemsemploying CDMA-technology, in particular if such systems also providethe possibility for handover to neighbor cells on other frequenciesand/or neighbor cells based on other radio access technologies.

In current UMTS Radio Access Networks based on the FDD mode, a known wayof handling a situation where there are too many neighbor cellsassociated with an Active set supporting communication between a mobilestation and the radio access network, is to order the mobile station toperform measurements on an arbitrary selection of said neighbor cells.

Published international patent application WO 98/36588 describes amethod and apparatus in a code division multiple access (CDMA)communication system for merging pilot neighbor lists to form a NeighborList Update message for transmission to a mobile station operating inthe CDMA-system. The mobile station measures pilot signal strengths andphase delays received at the mobile station. The mobile stationcommunicates the signal strengths and phase delays of all pilot signalsin the so called Active set and (optionally) Candidate Set to a MobileTelephone Switching Office (MTSO) in the CDMA-system. The MTSO retrievespredetermined neighbor lists of all Active/Candidate set pilots reportedby the mobile station and generates a neighbor list union as the unionof the predetermined neighbor lists. Any member in the neighbor listunion included in the Active/Candidate set reported by the mobilestation is removed and each remaining member, or base station, in theneighbor list union is operated on by a weighting function which takesinto account how often each member is included in the predeterminedneighbor lists of the reported Active/Candidate set pilots and thesignal strength of such pilots. The relative weight are then sorted andthe highest ranking members form a Neighbor List Update Message, up to apredetermined number of members.

U.S. Pat. No. 6,119,005 discloses a method and system for automateddetermination of handoff neighbor lists for base stations in a CodeDivision Multiple Access (CDMA) system. Mobile subscriber unitsoperating in the CDMA-system produces Pilot Strength Measurement (PSM)data that is indicative of the relative signal strength measured at themobile subscriber units from a plurality of pilot channels transmittedby base stations in the CDMA-system. The mobile subscriber unitstransmit this data along with a list of viable pilot channel candidatesas determined by the mobile subscriber units, to the base stationsserving the respective calls. For each base station in the CDMA-system,the system for automated determination of handoff neighbor listmaintains a data structure that stores data indicative of the number ofinstances that a pilot channel is recommended as a viable candidate bymobile subscriber units involved in calls served by the base station andthe sum of power levels that were measured by the various mobilesubscriber units for these instances. This data is processed todetermine a metric for each pilot channel entry in the data structuredata, said metric being a function of the number of instances that thepilot channel is recommended multiplied by a weighting factor summedwith the sum of power levels. Based on the determined metric values, thepilot channel entries are rank ordered and the resultant list istruncated to a predetermined number of entries forming an updatedNeighbor List for the base station at hand.

In published PCT/SE01/01528 a method and apparatus for ranking a set ofneighbor cells associated with a first cell in the cellular radiocommunication network are described. For each cell member in saidneighbor cell set, handover statistics reflecting how frequent said cellmember is involved in handovers aiming at modifying Active setsincluding at least said first cell to include said cell member areregistered. Based at least in part on the registered handover statisticsfor the different cell members in the set of neighbor cells, the cellmembers of said set of neighbor cells are ranked. The invention alsoincludes a method for controlling mobile station measurements whereinneighbor cells are selected for inclusion in a Monitored set based onsaid ranking.

The used concepts shall be consistent through-out the document. Thefollowing concepts-are supposed to be consistent with the 3GPPspecifications as far as possible:

Active Set: [3GPP] The cells involved in a radio connection (softhandover)

Neighbour Cell List: A list of defined neighbours to a cell in thenetwork. The Neighbour Cell List may consist of intra-frequencyneighbours (for soft handover), inter-frequency neighbours (forinter-frequency handover) and inter-RAT/GSM (for inter-RAT/GSMhandover). For convenience one could talk about three differentNeighbour Cell Lists (or Sublists).

Neighbour Set: The union of the cells in the Neighbour Cell Lists of thecells in the Active Set excluding the cells in the Active Set. Forconvenience one could talk about three different Neighbour Sets (orSubsets).

Monitored set: [3GPP] The cells the UE (User Equipment) is requested byUTRAN to measure (monitor). In general the Monitored set is a subset ofthe Neighbour Set. For convenience one could talk about three differentMonitored sets (or Subsets).

In UMTS, the UE is required to be able to measure the following numberof cells:

Active Set+Intra-frequency Monitored Subset<=32

Inter-frequency Monitored Subset<=32

Inter-RAT/GSM Monitored Subset<=32 (if GSM is supported) (For specifiedperformance only 8 intra-frequency cells need to be measured)

Unmonitored set: The cells in the Neighbour Set that is not included inthe Monitored set. For convenience one could talk about three differentUnmonitored sets (or Subsets).

Detected Set: [3GPP] Cells detected by the UE which are neither in theActive Set nor in the Monitored set. This means that cells in theDetected Set may or may not belong to the Unmonitored set. In 3GPP UTRANonly intra-frequency neighbours can belong to the Detected Set. This setis not further treated in connection with the present invention.

SUMMARY OF THE INVENTION

The problem dealt with by the present invention is to provide conditionsfor increasing the probability that the most suitable cells are includedin an Active set supporting communication between a cellular radiocommunication network and a mobile station.

While the above prior art method according to PCT/SE01/01528 solves theproblem to have cells which are very likely to become the best cell whenmoving around in the radio network in a monitoring list, it is alsoimportant to monitor cells which are unlikely to become strong butoccasionally could become strong and then start close monitoring whenthey have become strong. This is due to the fact that the stronger thecells the lower are the delays for detection.

Consequently there is a compromise to be done between the amount ofclosely monitored cells and using “strong cell detection” mechanism forcells.

The above given criterion “strong” indicates some measure on quality.The strongest cells are not necessarily the ones that have the highestquality measure for a given handover situation. For example, accordingto the above, the neighbouring cell list can include both inter-cell andintra-cell neighbours. An inter-cell neighbour, although it can havehigher signal quality than an intra-cell neighbour (and thus being“stronger”), it is less important for a cell in the Active Set. Besides,there is no quality measure associated with the neighbour cells. Cellsin the Neighbour Cell List are ordered randomly and thereafter selectedfrom the top of the list.

The selection of cells in the Monitored set is done by performingmultiple selection cycles in which members of the neighbor cell setsassociated with the Active set cells are considered for inclusion in theMonitored set.

The Detection set is created from the neighbouring cells to a certaincell in the Active set which are unlikely to become strong butoccasionally could become strong.

When one of the cells in the detection set has become strong, it isincluded into the Monitored set and vice versa.

Detected cells are, however, not included in the Monitored set unlessthey are put at the top of a Neighbour Cell List. The normal procedurewould be to include the cell in the Active set. The handling of reportedcells belonging to Monitored set or Detected Set is not treated in thisinvention. This invention is about how to create a Monitored set(Monitored Subsets) from the Neighbour Cell Lists (Neighbour CellSublists) of the cells in the Active set.

More specifically, the problem is solved using a method according toclaim 1 and its subclaims.

An advantage afforded by the invention is that it provides conditionsfor increasing the probability that the most suitable cells are includedin an Active set supporting communication between a cellular radiocommunication network and a mobile station.

Another advantage offered by the invention is that it enablesinterference reductions in a cellular radio communication system.

Yet another advantage afforded by the invention is that, when applied inan interference limited system, such as a system based on Code DivisionMultiple Access technology, it enables increased system capacity.

Still another advantage afforded by the invention is that there is noneed of ranking cells and no cells will be truncated if cells in thedetected set are included into the neighbouring cell list. Onlymeasuring the signal strength in order to determine the strength of thecells are needed.

The invention will now be described in more detail with reference toexemplary embodiments thereof and also with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a communication system including acellular radio communication network.

FIG. 2A are views illustrating an example scenario of how cells of thecellular radio communication network provide radio coverage in an areasurrounding a mobile station where the neighbouring cells areintra-frequency cells;

FIG. 2B are views illustrating an example scenario of how cells of thecellular radio communication network provide radio coverage in an areasurrounding a mobile station where the neighboring cells are both intra-and inter-frequency cells;

FIG. 3 is a table chart illustrating a basic method according to theinvention for only two cells in an Active set as shown in FIGS. 2A,2B.

FIG. 4 is a flow chart to illustrate the present method more in detail.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1 illustrates a non-limiting example of a communication system SYS1in which the present invention may be employed. The exemplarycommunication system SYS1 illustrated in FIG. 1 is a Universal MobileTelecommunication System (UMTS). The communication system SYS1 includesa cellular radio communication network NET, alternatively referred to asa Public Land Mobile Network (PLMN), and User Equipment (UE),alternatively referred to as mobile stations (MS).

The exemplary cellular radio communication network includes a corenetwork CN and two access networks, a UMTS Terrestrial Radio AccessNetwork (UTRAN) RAN1 and a GSM Base Station System (BSS) BSS1.

The core network CN includes a Mobile services Switching Center node MSCthat provides circuit-switched services and a General Packet RadioService (GPRS) node SGSN, sometimes referred to as a serving GPRSsupport node (SGSN), which is tailored to provide packet-switched typeservices.

The UMTS Terrestrial radio access network RAN1, referred to as radioaccess network RAN1 in the following, and the GSM Base Station System,referred to as base station system BSS1 in the following, both provideradio communication between the cellular radio communication network andmobile stations, such as mobile station MS in FIG. 1, but usingdifferent radio access technology. Thus the radio access network RAN1uses the new Wideband Code Division Multiple Access (WCDMA) radio accesstechnology, while the base station system uses GSM/EDGE radio accesstechnology which are both well known to a person skilled in the art.

The radio access network RAN1 includes one or more radio networkcontrollers (RNCs). For sake of simplicity, the radio access networkRAN1 of FIG. 1 is shown with only one radio network controller node RNC.Each radio network controller is connected to and controls a pluralityof radio base stations RBS1. The radio access network RAN1 is connectedto both core network nodes MSC and SGSN over a radio access networkinterface referred to as the Iu interface.

The base station system BSS1 includes one or more base stationcontrollers (BSCs). For sake of simplicity, the base station system BSS1of FIG. 1 is shown with only one base station controller node BSC. Eachbase station controller is connected to and controls a plurality of basetransceiver stations (BTSs) BTS1,BTS2 (only two shown in FIG. 1). Thebase station system BSS1 is connected to the mobile services switchingcenter MSC over an interface referred to as the A interface while thebase station system BSS1 is connected to the serving GPRS support nodeSGSN over an interface referred to as the Iu interface.

As previously indicated, mobile stations, such as mobile station MSshown in FIG. 1, may communicate with the cellular radio communicationnetwork NET either via the base station system BSS1 or via the radioaccess network RAN1 as shown. Communication between mobile stations andthe base station system BSS1 occurs over a radio interface referred toas the Um interface while communication between mobile stations and theradio access network RAN1 occurs over a radio interface referred to asthe Iu interface. The details of the. Um-interface are specified in the04- and 05-series of GSM Technical Specifications while the details ofthe Iu-interface are specified in the 24- and 25-series of UMTSTechnical Specifications.

In order to describe the method of the present invention, it is assumedthat the mobile station MS is communicating with, for instance, theradio base station RBS1 in RAN1 over a dedicated communication channel(a traffic channel) and with a base station RBS2 in RAN1 in a softhandover situation in the system RAN1 and will after that becommunicating with the radio base station RBS2. Thus, the presentinvention is primarily used when the mobile is connected in UTRAN (i.e.in this example system RAN1) and for making soft intra-frequencyhandover within UTRAN. However, the inventive method can also be usedfor hard handover between e.g. two intra-frequency cells or betweenGSM-interrupt systems.

FIG. 2A–2B illustrate an example scenario of how the exemplary cellularradio communication network NET provides radio coverage in ageographical area where a mobile station MS is currently located and fordifferent pairs of duplex radio frequency carriers. In this examplescenario, the radio access network RAN1 operates in the FrequencyDivision Duplex (FDD) mode of UTRAN and provides radio coverage usingcells assigned a first pair of duplex radio frequency (RF) carriers,i.e. an unlink RF carrier and a downlink RF carrier, as well as usingcells assigned a second pair of duplex RF carriers.

FIGS. 2A and 2B illustrates schematically cells C11–C22 in an active setof the radio access network RAN1 covering the area where the mobilestation MS is located using the first pair of duplex radio frequencycarriers. As illustrated the cells C11–C22 partly overlap each other andmay be of different size as well as different shape.

FIGS. 2A and 2B illustrates schematically cells C13–C18 and C31–C43,respectively of the radio access net-work RAN1 covering the area wherethe mobile station MS is located using the second pair of duplex radiofrequency carriers. In FIG. 2A and FIG. 2B, the geographical cellborders of cells C11 and C12 are illustrated as thin dashed lines.

As illustrated by FIG. 2A, both cell C11 and cell C12 in the active sethave a plurality of neighbouring cells, i.e. cells which are providingradio coverage in areas overlapping cell C11 and cell C12, respectivelyand cells which are providing radio coverage in areas adjacent to cellsC11 and C12 respectively. Thus, cells C13–C18 in FIG. 2A, cells C31–C38in FIG. 2B constitute a set of neighbor cells associated with cell C11while cells C13, and C18–C22 in FIG. 2A, cells C31–C34 and C38–C42 inFIG. 2B constitute a set of neighbouring cells associated with cell C12.Cells C11 and C12 do not by definition constitute any cells in theNeighbouring Set. In the neighbouring cell list (NS) for cell C11 alsocell C12 is included, and cell C11 is included in the neighbouring celllist for C12.

In the context of the present example scenario, the mobile station MS isassumed to be in a state of active communication with the cellularnetwork NET using a dedicated radio communication channel supported byan Active set consisting of cells C11 and C12. It is further assumedthat the mobile station MS is capable of communicating both according tothe Um interface, i.e. using the GSM air interface, as well as accordingto the Iu interface, i.e. using the UTRAN FDD mode air interface.

As previously discussed, the mobile station MS need to perform downlinkradio transmission measurements for the purpose of supporting handover.Thus the radio access network RAN1 orders the mobile station MS toperform measurements on downlink radio transmissions in the cells of theActive set, i.e. cells C11 and C12 in the present example scenario, aswell as in the cells of a so called Monitored set comprisingneighbouring cells to the cells in the Active set, i.e. cells which arelikely to become targets for handover in the future.

If the mobile station MS were to perform downlink measurements for cellsC11 and C12 in the Active set as well as all neighbor cells to cells C11and C12, the mobile station MS would need to perform measurements forall cells illustrated in FIGS. 2A,2B, i.e. at least 29 cells.

The current specifications for UTRAN FDD mode (see 3GPP technicalspecification 25.331) allows a radio access network to order measurementof downlink transmissions in up to 32 cells, each of these cellsoperating at the same downlink frequency as the cells of the Active set(Intra-frequency measurements). Cells operating at downlink frequenciesthat differ from the frequency of the cells in the Active set(Inter-frequency measurements) and cells using another Radio AccessTechnology (Inter-RAT measurements). Thus a mobile station may beordered to perform downlink transmission measurements for a total of3×29 cells.

The capacity for performing downlink transmission measurements may varyfrom mobile station to mobile station, but may in many instances besignificantly less than 32 cells each for Intra-frequency,Inter-Frequency and Inter-RAT measurements, e.g. in the order of 8–10cells for each different category, i.e. a total of 24–30 cells. If amobile station is ordered to perform too many downlink transmissionmeasurements, the rate at which the mobile station will be able toreport measurement results to the radio access network will be severelyreduced and may thus result in a significant delay before handover tochange a current Active set to a more suitable Active set can beperformed. This in turn causes increased interference levels due toradio communication between the mobile station and the radio accessnetwork occurring using unnecessarily high transmission power levels.The delayed handover may also result in a loss of communication in asituation where the radio environment changes very quickly. In order toeliminate or at least reduce the risk for ordering a mobile station toperform downlink transmission measurements in excess of its capacity,configuration parameters may be provided in a radio access networkenabling operation and maintenance personnel to control the maximumnumber of downlink transmission measurements in each category that theradio access network is allowed to order mobile stations to perform.

Assuming in the present example scenario that configuration parametersin the radio access network RNC specifies a maximum of 8 cells each forIntra-frequency, Inter-Frequency and Inter-RAT downlink transmissionmeasurements, i.e. a total of 24 cells, the mobile station MS can not beordered to perform downlink transmission measurements for all neighborcells to cells C11 and C12 in the current Active set.

In current UMTS Radio Access Networks, a known way of handling asituation where there are too many neighbor cells associated with anActive set supporting communication between a mobile station and theradio access network, is to order the mobile station to performmeasurements on an arbitrary selection of said neighbor cells. However,since only an arbitrary selection of all neighbor cells are measured,there is a significant risk that the most suitable cell, i.e. the cellwhere communication between the mobile station and the radio accessnetwork can be maintained using minimum radio transmission power levels,is never considered as a candidate for handover. Not using the mostsuitable cell for radio communication between the radio access networkand the mobile station means that said radio communication occurs usingunnecessarily high transmission power levels, which causes increasedinterference. The increased interference in turn translates to reducedsystem capacity.

The present invention deals with situations as the one illustrated inthe example scenario above by providing conditions for increasing theprobability that the most suitable cells are included in an Active setsupporting communication between a cellular radio communication networkand a mobile station when the capacity for performing downlink radiotransmission measurements is limited.

FIG. 3 shows a set of tables to illustrate the present method. In orderto simplify the description and FIG. 3, only two cells C11 and C12 arein the active set as illustrated in FIGS. 2A–2B. Furthermore, theneighbouring cells to cells C11 and C12 illustrated in FIGS. 2A,2B arecontained in FIG. 3.

The cells in the active set are listed in quality order. Theirneighbours have been listed in Table 1 in arbitrary order i.e. it is notnecessary to range the neighbouring cells for each cell in the activeset according to best signal strength (“strongest cell”-order) but canbe ordered randomly. Among the neighbouring cells for each cell in theActive set several cells can be common as will be mentioned below. Thevarious neighbouring cells have been taken from FIGS. 2A–2B and in Table1 it is indicated the neighbouring cell to the respective cell C11,C12in the Active set and in parenthesis the FIG. 2A–2B, where therespective cell is shown.

Table 1 thus shows the list of the neighbours (Neighbouring cell List)to each cell C11,C12 in the Active set, cell C11 being ranked withhigher quality than cell C12. In this case only two columns ofneighbouring cells to the cells C11 and C12 are shown in FIG. 3.

To create the monitoring list according to the invention, one starts topick the first cell on top of the neighbouring cell list belonging tothe active cell C11 with the highest quality. This list includes, asshown in FIGS. 2A–2B, 6+9=15 neighbouring cells if also cell C12 isincluded. A less number of neighbouring cells are chosen than isincluded in Table 1, for instance, only one cell. The cells C12,C31 andC18 are put into the monitoring list shown in Table 2.

Next, the cell C12 in the Active set is investigated. This cell C12 hasas shown in FIGS. 2A–2B, 6+10=16 neighbouring cells if also cell C11 isincluded as neighbouring cell to C12.

When the first group of neighbouring cells have been selected, a newselection is started from the neighbouring cells to cell C11. Onecontinues to pick the first cell C32 from the top of the neighbouringlist belonging to the cell C12 with the next highest quality.

When the first one of the cells on the top of the Neighbouring list forC11 and C12 have been selected, the next one in the neighbouring celllist associated with the active cell C11 with the highest quality isselected, i.e. cell C31 to cell C11 in Table 1, FIG. 3, thereafter cellC11 to cell C12. Next is cell C18 for cell C11, cell C14 for cell C12and this procedure is going on until the Monitored set is full.

Table 2, FIG. 3 is the list created after eight steps according to thisexample. Neighbouring cells in the active set have been excluded (cellsC11,C12 in this example) and cells which are counted twice are indicatedin bold (cells C13,C14,C18,C32, C31,C34 in this example). This meansthat the number in Table 2 is the configured max number. The reductionto Table 3 in order to include a cell only once will mean that thenumber in Table 2 is less than the configured max number. The number inTable 2 can however be chosen greater than the configured max number. sothat the reduction to Table 3 in order to include a cell only once willresult in that the number in Table 3 is equal to the configured maxnumber.

The procedure shown in the flowchart of FIG. 4 which will be describedbelow, takes care of this by filling the Monitored set with cells thatis not already in the set until the set is filled up to the configuredmax number.

The resulting Monitored set is shown in Table 3. In this table all theneighbouring cells have been collected and put together and none ofthese neighbouring cells are counted more than once.

When the resulted Monitored set has been created, it will be sent downto the mobile station MS to initiate the measurements and measurementreport messages. The mobile is also ordered to measure (signal strength)on cells that are not included in the Monitored set, the so calleddetected set. The cells in the Monitored set will be detected quicklyand with low spread in delay by the mobile, the cells in the detectedset will typically be slower with higher delay spread.

If the radio network controller RNC in FIG. 1 now gets a measurementreport it will map the report to a cell in either the monitored or thedetected set and appropriate measures can be taken. The UE can berequested to let cells in the Detected Set (only intra-frequency cells)trigger a report rather than be ordered to measure on cells not in theActive set or the Monitored set. A reported cell will be checked if itbelongs to the Active set, the Monitored set or the Unmonitored set. Ifthe cell does not belong to any of these sets, it will be discarded (andthe connection may be released).

The measures are: Include (add or replace) or remove an intra-frequencycell in the Active Set. Do inter-frequency or inter-RAT/GSM handover toan inter-frequency or an inter-RAT/GSM cell.

If the cell in the report can not be identified it shall be discarded.

FIG. 4 shows a flow chart to illustrate the inventive method more indetail. From the beginning it is assumed that there are a certain numberof neighbouring cells in a Neighbouring cell list NS (including one ormore active cells) to an active cell in an Active set AS. These cells inNS are not ranked but can be randomly ordered with regard to theirimportance for handover, see above.

At first, the cells in an NS are ranked according to signal qualityprovided by the associated active cell in AS, block 1. This is done bymeans of ordinary quality measurements by the user equipment UE (mobilestation MS in FIG. 1).

Thereafter a cell in the Active set AS having the best signal quality isconsidered (e.g. cell C11 in FIG. 3) and a first member of the cell(cell C12 in FIG. 3) in the associated NS is considered, block 2. Ifthen this cell in NS is a member of the set AS, block 3, all the stepsaccording to blocks 4,5 and 6 are neglected. If the cell was not amember of the Active set AS (as is the case for cell C31, FIG. 3 nexttime), it is investigated, block 4, whether it is already in theMonitored set, and if “No” it is included in the Monitored set, block 5.

The normal procedure for a considered cell in the NS is:

“No” from block 3, “No” from block 4, “No” from block 5 and “No” fromblock 6.

Next, it is investigated whether the neighbouring cells in NS to thenext cell (C12 in FIG. 3) in the Active set AS has been considered,block 7. If not, “Yes” (the most frequent case) the members of that NSis considered by repeating the procedure for this cell member asindicated by “A” in FIG. 5. If on the other hand all the cells in theActive set AS have been considered, “No” from block 7, it isinvestigated if all cells in the NS with higher quality have beenconsidered, block 8. If at least one such NS-cell remains, “Yes” atblock 8, that cell is investigated in accordance with the previous steps(blocks 3,4,5, etc) as indicated by “A” in FIG. 5. In FIG. 3, thiscorresponds to “going back” from cell C12, group G21 to cell C11 andscanning the next group G12 of neighbouring cells.

If on the other hand there is no such cell left, “No” at block 8, thenit investigated if all cells in NS have been considered, block 10, andif they have, “Yes”, the selection process is stopped, otherwise (“No”)the next cell member in the NS under consideration is considered and theprocedure (blocks 3,4,5, etc) is run through as indicated by “A”.

Summarised, the ordinary procedure for a neighbouring cell set NS for agiven cell in AS is the steps according to blocks 3,4,5,6 and 7; and

if there is a new neighbouring cell in the same list NS as before, thisnew cell for the same active cell is to be considered, “NO” at block 7and the procedure is repeated for this next neighbouring cell in thelist, block 9, A and back to block 3 again; or

if a new list NS for the next active cell in AS (having lower quality)is to be scanned “YES” at block 7, the procedure is repeated, block 8 A,back to block 3 and so on.

Block 3 “YES” eliminates all the cells in NS which are cells in theActive set, and block 4 “YES” eliminates all the cells in NS which havebeen counted twice, c.f. Table 2 in FIG. 3.

This goes on until all the cells in NS for a given cell in AS have beenconsidered. The procedure is repeated as many times until the Monitoredset MS has reached its maximum size (block 6, “Yes”).

Thus, block 12, i.e. taking cell(s) from a Neighbour Cell List of a cellin the Active Set with lower quality, and block 9, i.e. taking cell(s)from a Neighbour Cell List of a cell in the Active Set with highestquality after that all lists have been considered in one cycle, and anew scanning of neighbouring cells to a cell in AS with lower quality toscanning the NS-cells to an AS-cell with higher quality; respectivelyare the essential features of the invention.

Blocks 10 and 11 takes care of the case when a certain neighbouring celllist NS is longer than the other lists and represents a local loop forthis list.

It is not abolutely necessary to include one or more active cells in thethe Neighbour Cell List NS shown in FIG. 3. The list can include only“true” neighbouring cells such as C13–C18, C31–C38 in FIGS. 2A,2B forthe active cell C11.

When the Monitored set has been created, it will be sent down to themobile station MS in FIG. 1 to initiate the measurements and measurementreports. The mobile is also ordered to measure on cells that are notincluded in the Monitored set MS, the so called detected set.

The cells in the Monitored set will be detected fast and with low spreadin delay by the mobile, the cells in the detected set will typically beslower with higher delay spread.

When the radio network controller RNC gets the measurement report fromthe mobile MS it will map this report i.e. by comparing with theMonitored set of a cell in the Active set. If there are one cell (ormore cells) which cannot be identified this cell (or cells) will bediscarded and a new list of neighbouring cells is sent to the mobilestation MS to be used by this when measuring the signal strength. Bythis, always the best list of neighbouring cells will be available forthe mobile station MS and no need of ranking the neighbouring cells isnecessary and no cells will be truncated as in the prior method.

In embodiments of the invention implemented in cellular radiocommunication networks, such as GSM networks, not supportingmacrodiversity/soft handover, the Active set supporting communicationbetween the network and a mobile station always includes one cell, i.e.the serving cell.

As a person skilled in the art appreciates, application of the inventionis in no way limited to only cellular radio communication networksconforming to the UMTS specifications. The invention is generallyapplicable to all cellular systems in which the cellular radiocommunication network orders mobile stations to perform downlinkmeasurements on transmissions in neighbor cells to the cells currentlyserving said mobile stations. Thus, the invention is also applicable incellular radio communication networks adhering to e.g. GSM, PDC,TIA/EIA-136 and CDMA2000 specifications.

1. A method in a cellular radio communication network for determining amonitored set from a list of neighboring cells to each of at least afirst and a second active cell in an active set of such cells for a userequipment in said network, said first and second active cell togetherwith the associated neighboring cells being ranked according to signalquality of said active cells in the active set, said method comprisingthe steps of: a) considering a first member in said neighboring listassociated with the active cell in said active set having the bestquality; b) including said first member in said monitoring list if notalready a member in said list; c) consider a first member of next lowerranked cell in said active set for selection if not considered before;while if said first member of next lower ranked cell in said active sethas been considered, d) consider the next member in said neighboringlist associated with the cell in said active set having the bestquality; and e) continuing selecting cells according to steps a)–c)until said monitored set has reached its maximum size.
 2. The methodaccording to claim 1, wherein at least one active cell is included insaid list of neighboring cells to a certain other active cell.
 3. Themethod according to claim 1, further comprising the step ofinvestigating whether a member of the neighboring cell set underconsideration is already in the monitored set and, if so, excluding thismember from consideration under step c).
 4. The method according toclaim 1, wherein said neighboring cells are intra-frequency cells. 5.The method according to claim 1, wherein said neighboring cells areinter-frequency cells.
 6. The method according to claim 1, wherein saidneighboring cells are inter-RAT/GSM cells.
 7. A method in a cellularradio communication network for determining a monitored set from a listof neighboring cells to each of at least a first and a second activecell in an active set of such cells for a user equipment in saidnetwork, said first and second active cell together with the associatedneighboring cells being ranked according to signal quality of saidactive cells in the active set, said method comprising the steps of: a)considering a first member in said neighboring list associated with theactive cell in said active set having the best quality; b) includingsaid first member in said monitoring list if not already a member insaid list; c) consider a first member of next lower ranked cell in saidactive set for selection if not considered before; while if said firstmember of next lower ranked cell in said active set has been considered,d) consider the next member in said neighboring list associated with thecell in said active set having the best quality; and e) continuingselecting cells according to steps a)–c) until said monitored set hasreached its maximum size; wherein at least one active cell is includedin said list of neighboring cells to a certain other active cell; and,further comprising the step of investigating whether a next member ofnext lower ranked neighboring set is a member of the active set and, ifso, excluding this member from consideration according to step c).